The disclosure relates to centrifugal compressors. More particularly, the disclosure relates to electric motor-driven centrifugal compressors having controllable inlet guide vane arrays.
One particular use of electric motor-driven centrifugal compressors is liquid chillers. An exemplary liquid cooled chiller uses a hermetic centrifugal compressor. The exemplary unit comprises a standalone combination of the compressor, the condenser unit, the cooler unit, the expansion device, and various additional components.
Some compressors include a transmission intervening between the motor rotor and the impeller to drive the impeller at a faster speed than the motor. In other compressors, the impeller is directly driven by the rotor (e.g., they are on the same shaft).
Centrifugal compressors have a limitation for operating at high head due to flow instability called “surge”. The effect of surge is high vibrations and damage to the impeller and bearings. Hence, it is preferable to avoid surge in a compressor. At a given speed, a compressor has an associated maximum head limitation dictated by surge. For a compressor having inlet guide vanes for capacity control, at a constant suction pressure the discharge pressure at which surge occurs reduces with load. Surge may be triggered by something which increases saturation temperature at the condenser or decreases saturation temperature at the evaporator. For example, if condenser water temperature increases, surge may occur.
In operation, the controller controls vane condition (orientation) to throttle the inlet flow to maintain a required amount of refrigerant flow rate.
Surge can be an issue at compressor startup. Surge may be limited by closing the inlet guide vanes at startup. In one baseline system, the compressor is operating at a given condition when the system suddenly shuts down (e.g., upon power loss, thus leaving the guide vanes in their pre-shutdown condition). At startup, to prevent surge, the guide vanes are first fully closed prior to starting the motor.